Among all defining characteristics of a solid-state image sensor technology, photo- and spectral-sensitivity are vital in determining the breadth of end-user applications. Concomitant single-photon detection with multispectral response stands as the ultimate goal within this space, but achieving both in a single, monolithic device presents a substantial challenge. To detect single photons with high fidelity—a high signal-to-noise ratio—detectors must provide large electrical gain in the optical-to-electrical conversion process and the false count events generated through on-chip electroluminescence, known as optical crosstalk, must be kept at a minimum.
In essentially all semiconductor photodetectors of visible or infrared light, each incident photon creates only a single photo-excited electron by the photoelectric effect. Spectral response is typically limited as well, with either device structure or material band gap preventing detection outside of a narrow design band. The few prior-art technologies which do provide a wide spectral response do not have single-photon sensitivity and those that exhibit single-photon sensitivity are generally confined to either visible through near-infrared (Vis-NIR) or short-wave infrared (SWIR) wavelength ranges. One technology that nearly satisfies both criteria at present, with prior-art devices demonstrating the capacity for either wide spectral response or single-photon sensitivity (but not both in the same device), is a type of semiconductor photodetector known as the avalanche photodiode (APD).
APDs provide optical-to-electrical gain by exploiting carrier multiplication through the impact ionization process. They are designed so that each photo-excited charge induced in the absorption region of the device is injected into a multiplication region where this charge is accelerated by a large electric field. When the injected charge reaches a sufficiently high kinetic energy, it can generate an electron-hole pair through an inelastic collision with lattice atoms in a process referred to as “impact ionization.” These newly liberated carriers are then also accelerated, and the process continues to create an “avalanche” of charge until all carriers have exited the high-field multiplication region of the device.
At a sufficiently large electric-field intensity known as the “avalanche breakdown field,” there is a finite probability that the avalanche multiplication process can lead to a self-sustaining avalanche. By applying a field larger than the breakdown field, the APD is operated in a metastable state in which the injection of a single photo-excited charge can trigger the build-up of an easily detectable macroscopic pulse of charge in an extremely short (c.a., <1 nanosecond) avalanche build-up period. This so-called “Geiger-mode” operation can provide high-efficiency detection of single photons. Devices operated in this regime are referred to as Geiger-mode APDs (GmAPDs). As those skilled in the art will appreciate, there are certain slight structural differences between a conventional APD and a GmAPD.
Although GmAPDs presently provide the highest internal gain of any solid-state detection technology, their spectral response is limited in the same manner as conventional semiconductor detectors: by device design and absorber band gap. GmAPDS built on silicon are considered a mature technology with device designs that enable wavelengths as short as 400 nanometers (nm) to reach device active areas for detection. Unfortunately, the 1.12 eV indirect band gap of silicon at room temperature limits its detection of longer wavelengths to the NIR (<1000 nm).
In order to enable detection of SWIR wavelengths (i.e., >1000 nm), GmAPDs must instead be built upon materials with smaller band gaps. But working with alternative materials comes at a significant cost in process and product maturity, as all silicon technologies benefit from its decades atop the global semiconductor marketplace. So although alternative materials must be used to obtain SWIR sensitivity, present designs, which in turn are dictated by material structure and fabrication capabilities, typically result in visible and NIR photons being absorbed before reaching the device active regions which register detection events.
GmAPDs of various materials have been used in each pixel of a focal plane array to create imagers with single photon sensitivity. One prior-art approach uses these GmAPD arrays to simply count every time a photon strikes a given pixel to build up an intensity image based on the number of counts per pixel in a given integration time. (This is essentially a digital analogue to the less sensitive analog imagers that involve the capacitive integration of photo-induced current.)
An unwelcome consequence of this extreme sensitivity to external photons is an extreme sensitivity to thermal generation of electrons and holes—so-called “dark counts”—within the depletion region. There is also an additional generator of dark counts that is unique to the focal plane array geometry; namely, electroluminescence from active regions during avalanche events, which cause spurious dark counts in neighboring pixels. In some instances, the emitted photons travel directly to their nearest neighbors, while in others, the photons will first travel towards the back side of the material substrate before reflecting back towards further distant neighboring pixels. These spurious dark counts reduce spatial and temporal image fidelity.